Popup to insert into database doesn't work (PHP)

I wanna make a popup windows to add images into database, but the problem this popup doesn't work.

when I click into submit button I didn't show any result (nothing) Like when I click into close button

This's popup code :

<div class="modal fade" id="myModal" role="dialog">
<div class="modal-dialog">

<div class="modal-content">
<div class="modal-header">
<button type="button" class="close" data-dismiss="modal">&times;</button>
<h4 class="modal-title">Ajouter Photo</h4>
</div>
<div class="modal-body">
<form action="gallery.php"  method="post" class="form-horizontal" role="form" enctype="multipart/form-data">
<div class="form-group">
<label  class="col-sm-2 control-label"						for="inputEmail3">Titre</label>
<div class="col-sm-10">
<input type="text" name="titre" class="form-control"		 placeholder="Titre"/>
</div>
</div>
<div class="form-group">
<label class="col-sm-2 control-label"
for="inputPassword3" >Photo</label>
<div class="col-sm-10">
<input type="file" name="image" class="form-control" />
</div>
</div>

<div class="form-group">
<center>
<input type="submit" name="submit_post" class="btn-default" Value="ajouter">
</center>
</div>
</form>
</div>
<div class="modal-footer">
<button type="button" class="btn btn-default" data-dismiss="modal">Fermer</button>
</div>
</div>

</div>
</div>

<?php echo $result; ?>

<?php
$sql_gallery = "SELECT * FROM media WHERE Value='galerie'";
$sql_gallery_query = mysqli_query($conn,$sql_gallery);
$n = mysqli_num_rows($sql_gallery_query);
if ($n == 0){
echo '
<div class="typo-wells">
<div class="distracted">
<h3 class="ghj">0 Photos ajouter</h3>
<center>
<div class="well">
 There isnt any picture here !!
</div>
</center>

<center>
<button type="button" class="btn btn-info btn-lg ajouter" data-toggle="modal" data-target="#myModal">Ajouter</button>
</center>
</div>
</div>

<div class="clearfix"> </div>
<br><br><br><br><br><br><br>

';

}

?>

And this's the php code to insert into database

$result ='';

if(isset($_POST['submit_post'])){
$titre = strip_tags($_POST['titre']);

if(isset($_FILES['images'])){

$image_name = $_FILES['image']['name'];
$image_tmp = $_FILES['image']['tmp_name'];
$image_size = $_FILES['image']['size'];
$image_ext = pathinfo($image_name, PATHINFO_EXTENSION);
$image_path = '../images/'.$image_name;
$image_db_path = 'images/'.$image_name;

if($image_size < 1000000000){
  if($image_ext == 'jpg' || $image_ext == 'png' || $image_ext = 'gif'){
  if(move_uploaded_file($image_tmp,$image_path)){
  $ins_sql = "INSERT INTO media (Images_md,Value,Title) VALUES ('$image_db_path','galerie','$titre')";
  if(mysqli_query($conn,$ins_sql)){
  $result = '<div class="alert alert-success alert-dismissable">
<button aria-hidden="true" data-dismiss="alert" class="close" type="button"> × </button>
Success! Well done its submitted. </div>';
  }else {
  $result = '
<div class="alert alert-danger alert-dismissable">
<button aria-hidden="true" data-dismiss="alert" class="close" type="button"> × </button>
Error ! Change few things. </div>';
 }
 }else {
 $result = '
<div class="alert alert-danger alert-dismissable">
<button aria-hidden="true" data-dismiss="alert" class="close" type="button"> × </button>
Sorry, Image hos not been uploaded. </div> ';
  }
 }
else {
  $result = '
<div class="alert alert-danger alert-dismissable">
<button aria-hidden="true" data-dismiss="alert" class="close" type="button"> × </button>
Please Add a jpg OR png OR gif Image. </div>
';
     }
  }else {
  $result = '
<div class="alert alert-danger alert-dismissable">
<button aria-hidden="true" data-dismiss="alert" class="close" type="button"> × </button>
Image File Size is much bigger then Expect. </div>
				';
			}

		}

	}

Ajax code :

$(document).ready(function(){
$(document).on('submit', '#idform', function (e) {
 e.preventDefault();
 var form = $(this);

 $.ajax({
		method: $(this).attr('post'),
		url: $(this).attr('gallery.php'),
		data: $(this).serialize(),
		dataType: "json",
			beforeSend: function () {
				form.css('opacity', '0.0');
					},
 complete: function () {
 form.css('opacity', '1');
 },
 success: function (data, status) {
	 alert('Photo added');
 },
 error: function (data, status) {
alert('error');
 }
 });

 });
 });

When I click in add button I saw ' Error ' , Can you give me a solution ?

I search a lot without any result :'(